The present invention relates to a radiation-sensitive mixture that is positive-working, i.e., the mixture becomes soluble as a result of irradiation; that contains, as the essential constituents,
(a) a compound that forms a strong acid under irradiation,
(b) a compound with at least one C--O--C bond cleavage by acid,
(c) a binder which is insoluble in water and soluble in aqueous-alkaline solutions, and
(d) a dye;
and that is suitable for the preparation of photoresists, electronic components and printing plates, as well as for chemical milling. Mixtures of this general description have been described, for example, in U.S. Pat. No. 3,779,778, No. 4,101,323 and No. 4,189,323, in German Offenlegungsschriften No. 2,718,254 and No. 2,928,636 and in European patent applications No. 0 006 626, 0 006 627 and 0 022 571. Further improvements and variants are the subjects of European patent application No. 0 042 562 and German Offenlegungsschrift No. 3,151,078.
When these materials are exposed, an acid is formed by photolysis of the compound (a), which acid effects cleavage of C--O--C groups of compound (b); as a result, the exposed areas of the light-sensitive layers become soluble in the developer. The light-sensitive mixtures can additionally contain soluble dyes or fine particles or dispersible dyes and, depending on the application, UV absorbers. Dyes which have proved suitable are the triphenylmethane dyes, particularly in the form of their carbinol bases, which show a marked image contrast not only after development, but even after exposure.
It is known that triarylmethane dyes generally are used in certain radiation-sensitive mixtures as sensitizers for the absorption range of 550-820 nm. An improvement in light sensitivity of particular radiation-sensitive mixtures, achieved by addition of their leuco or carbinol bases, is described in German Patentschriften No. 2,608,082 (corresponding to U.S. Pat. No. 4,218,247), No. 1,117,391 (corresponding to U.K. patent application No. 944,126) and No. 2,526,720 (corresponding to U.S. Pat. No. 4,063,948), in German Offenlegungsschriften No. 2,817,428 (corresponding to U.S. Pat. No. 4,252,880) and No. 3,240,935, and European patent application No. 0 102 745 for dyes that belong to the triarylmethane class but that can also be derived from substituted benzo[a]carbazoles.
In accordance with U.S. Pat. No. 3,109,736, the leuco bases of triphenylmethane or diphenylmethane dyes are added for sensitizing light-sensitive mixtures. It has been found, however, that such radiation-sensitive mixtures cannot be stored, since the leuco base is oxidized to the dye even in the dark, i.e., even the background becomes deeply colored.
The use of triphenylmethane dyes in the positive-working radiation-sensitive mixtures mentioned above, which in most cases will be termed "acid-cleavable systems" below, are described in European patent application No. 0 042 562 and in German Offenlegungsschrift No. 3,151,078. The drawbacks described in U.S. Pat. No. 3,109,736 also manifest themselves in these mixtures if triarylmethane dyes are used. Moreover, the requirement for durable image contrast after exposure is not always fulfilled. It has also been found that the color constancy is inadequate in liquid mixtures, i.e., in coating solutions and so-called liquid resists, in part depending on the nature and purity of the solvents used.
Apart from triarylmethane dyes, polymethine dyes, which are classified as photographic dyes and are technically used in the photographic industry, can generally also be used in light-sensitive mixtures. As additives in photographic silver halide emulsions and for the other light-sensitive substances, such as zinc oxide, titanium(IV) oxide and organic photoconductors--which are sensitive only in the (ultra)violet range--these dyes must meet the following requirements:
When they are used as sensitizers, the dyes extend the spectral sensitivity of silver halide emulsions, which are sensitive only in the (ultra)violet range, into the visible, and sometimes the infrared spectral region.
When the dyes are used as antihalation dyes, they exert the function of filter dyes which are used for the absorption of undesired wavelengths, particularly in the UV region. This property simultaneously prevents the dispersion of light, caused by the photographic emulsion, and a resulting reduction in image sharpness.
The principle of adding photographic dyes of this type to positive- or negative-working reprographic mixtures is known for some radiation-sensitive mixtures. The objective in these instances is primarily the sensitization of the light-sensitive material, also in the visible spectral range. For these systems, which do not, however, correspond to the acid-cleavable systems described above, the sensitization ranges with respect to their wavelength are identical, or at least largely coincident with the absorption regions of the photographic dyes.
For cyanine dyes, this spectral region is generally in a range from 350-450 nm.
U.S. Pat. No. 3,106,466 discloses, for example, a copying material for the preparation of color-forming materials, photoresists and printing plates based on halogenated hydrocarbons, by means of which high image sharpness can admittedly be obtained after the addition of cyanine dyes; however, the mixtures show a relative insensitivity to light. It is stated that merocyanine dyes would be preferable in these cases.
A sensitizer which has the structural features of both cyanine dye and a merocyanine dye is described in PCT application No. 83/00,752, with an electrophotographic material as an example. The sensitizer is said to have good absorption in the visible region and a high stability of he exposed dye.
In German Patentschriften No. 1,283,093 and No. 1,286,898, trinuclear basic merocyanine dyes are used for sensitizing light-sensitive materials--which in this case, however, are color forming--based on halogenated hydrocarbons and N-vinyl compounds. These mixtures are said to have a high light sensitivity, intense color, high image sharpness and good stability.
European patent application No. 0 110 165 discloses a positive-working mixture for the preparation of a liquid resist, based on an alkali-soluble binder and an o-quinone-diazide, wherein the absorption region of the mixture can be shifted into a region below 350 nm, (that is, into the shorter wavelength region) by the use of polymethine dyes, the conjugated system of which is extended symmetrically by means of nitrones. These dyes also effect a high image sharpness contrast.
It is also known to obtain, by combination of several dyes, so-called panchromatically sensitized recording materials which allow sensitization across the entire spectral range. Such systems are described by referance to electrophotographic materials in German Offenlegungsschriften No. 2,817,428 and No. 2,353,639 and in European patent application No. 0 004 944.
In German Patentschrift No. 2,718,259 (corresponding to U.S. Pat. No. 4,189,323), light-sensitive mixtures are described that contain halogen-containing compounds which release acid under radiation, for example, trichloromethyl-triazine derivatives; a compound with at least one C--O--C bond which can be cleaved by acid; a binder; a ketone as solvent; and triarylmethane dyes, such as crystal violet, for effecting color changes. Examples 3 and 4 of this patent document show that it is also possible to use as the dye a styryl dye containing a benzo-fused heterocyclic ring or an nonsymmetrical cyanine dye. A sensitizing effect is not mentioned in this case.
Apart from the above-mentioned dyes, hemicyanines (German Offenlegungsschrift No. 3,410,387), oxadiazoles (European patent application No. 0 188 766) and hemioxonols or oxonols (European patent application No. 0 146 411 and German Offenlegungsschrift No. 3,319,991) can in principle also be used for sensitizing light-sensitive mixtures based on photopolymerizable compounds in the region 350-400 nm.
European patent application No. 0 146 411 describes positive- and negative-working light-sensitive mixtures that contain, in addition to an onium salt and a binder, a proton donor such as an o-quinone-diazide; but the disclosed class of proton donors also includes the leuco bases of dyes which are oxidized to the dye by the sensitized onium salt. It is even stated in this patent document that all onium salts are suitable in principle for absorption in the visible region or for sensitization, because protonation or a positive charge generally leads to a bathochromic color shift. In this regard, iodonium, sulfonium, bromonium, chloronium, phosphonium, sulfoxonium, oxysulfonium, selenonium, telluronium and arsonium salts, cyanine dyes with a positive charge due to quaternization and also anionic, (negatively charged) oxonol dyes, as well as neutral merocyanine dyes, are mentioned.
Such a generalization, heretofore conventional, which allows conclusions based on the absorption spectrum of a dye to be drawn with regard to the sensitization range of that dye, is put into question by German Offenlegungsschrift No. 3,319,991. The subject of this patent document is a negative-working light-sensitive mixture containing a diazo resin and, if appropriate, a binder, which mixture is to be sensitized in the region 350-450 nm by means of oxonol dyes. With these dyes, adequate adsorption of stray radiation, which allows improved image contrast even after relatively long irradiation, and higher image sharpness are achieved, in addition to high sensitization. By contrast, mixtures incorporating the dyes that are used in Comparison Example 1 of this patent document, such as benzeneazophenylamine, chrysoidine MC crystal and a hemioxonol dye, show a decrease in sensitivity, albeit with the same image sharpness, even though the dyes used ought to be suitable for sensitization on the bases of their absorption spectra.
The state of the art, as summarized above, therefore does not make it readily possible to predict whether a dye will be suitable as a sensitizer for specific light-sensitive mixtures.